It is known that polymaleimides can be utilized for the preparation of various polyaddition and polymerization products. Particular emphasis has been placed on bis-maleimide materials which exhibit thermal stability and good mechanical properties and, thus, are being more frequently utilized in high performance composite applications, such as electronic circuit board applications.
The currently used bis-maleimide systems include aromatic amines or alkenyl phenols as coreactants. U.S. Pat. Nos. 3,658,764 and Re. 29,316 are examples of patents that disclose reaction products of unsaturated bis-imides and amines. U.S. Pat. Nos. 4,100,140; 4,127,615; 4,130,600; 4,131,632; 4,371,719; and 5,077,363 are examples of patents which disclose crosslinked polymers resulting from the reaction of polymaleimides with alkenyl phenols or alkenyl phenol ethers optionally in the presence of epoxy resins. U.S. Pat. No. 4,038,251 discloses polyamide/polyhydric phenol reaction products prepared in the presence of amines.
A standard polyimide product conventionally used for various pre-peg applications comprises the reaction product of bismaleimidodiphenyl methane with methylene dianiline. The product is supplied as a solid powder to be dissolved in N-methylpyrrolidone for prepreg use. It has been noted, however, that the product is soluble only in the above-noted solvent or similar high boiling, difficult to process aprotic solvents. Correspondingly, solution stability of these systems is limited as evidenced by rapidly occurring precipitation and viscosity increases on standing. These factors necessitate that the prepolymer be formulated immediately prior to use.
Improvements in the above systems are noted in the products disclosed by U.S. Pat. Nos. 4,100,140 and 4,371,719, as cited above. U.S. Pat. No. 4,100,140 discloses improved products, based primarily on the reaction of bismaleimidodiphenyl methane and diallyl bisphenol A prepared in the optional presence of amine polymerization catalysts, which exhibit improved thermal, chemical and mechanical properties. Still further, U.S. Pat. No. 4,371,710 discloses improved products, based primarily on the reaction of maleimides with phenol containing at least one propenyl group, with the advancement being that products containing a propenyl-substituted phenol cure at a faster rate than those containing an allyl-substituted phenol. However, both of the aforementioned systems still have certain disadvantages in terms of storage-stability and limited solubility. For example, the systems are insoluble in conventional ketonic solvents and, when stored in higher boiling solvent solution, precipitate out of solution after short periods of time. Consequently, the products normally must be stored at refrigerated temperatures due not only to the instability of the solution but also to advancement of the resin systems at higher temperatures. Accordingly, their applicability for use in making solvent impregnated laminates is limited.
Elimination of some of the disadvantages as mentioned above is described in U.S. Pat. No. 5,189,128 wherein polymaleimide prepolymers are obtained by reaction of a polymaleimide with an alkylphenol or alkenyl phenol ether in the presence of specified molar amounts of a basic catalyst at elevated temperatures in order to obtain a predetermined amount of resin advancement. The resulting resin solutions exhibit improved solubility in relatively low boiling solvents such as methyl ethyl ketone and propylene glycol methyl ether and improved resin solution stability as evidenced by an absence of precipitation of the resin from solution. However, difficulty in obtaining a storage-stable advanced bismaleimide prepolymer product may still be encountered when using these resin solutions due to further polymerization of the prepolymer at room-temperature conditions. More importantly, a storage-stable prepreg product cannot be manufactured therefrom, resulting in a decrease in resin flow in the lamination process as the storage age increases. If the resin flow decreases too far, the bond strength will decrease, which leads eventually to delamination. Moreover, as the resin flow increases over time during storage, the manufacturing process must be accordingly readjusted. Thus, it becomes very important to the user of the prepreg product to be able to store said product without reducing resin flow. Generally, refrigeration must be used, which is an additional expense and not always available to users of the prepreg product.
U.S. Pat. No. 5,077,363 discloses a heat-curable bismaleimide prepolymer resin containing bismaleimide, alkenylphenol and a specified amount of N-vinylpyrrolidone, whereby addition of the N-vinylpyrrolidone results in a low viscosity resin solution and an improvement in tack for prepreg products made therefrom. The unpolymerized mixtures containing the bismaleimide, the alkenylphenol and the N-vinylpyrrolidone, that is before any resin advancement whatsoever occurs, may additionally contain inhibitors such as hydroquinone, benzoquinone and phenothiazine. Such inhibitors may be added to the unreacted mixture to control or slow the rate of polymerization once polymerization is initiated. However, the advanced polymaleimide products obtained therefrom do not result in storage-stable prepreg products manufactured therefrom and hence, lead to the same problems as described hereinabove.
Similarly, it is noted in U.S. Pat. No. 4,371,719, discussed supra, that inhibitors may be added to unpolymerized mixtures containing a maleimide and propenyl-substituted phenol in order to retard the degree of resin polymerization. Unfortunately, the disadvantages encountered above, namely failure to achieve a storage-stable advanced polymaleimide prepolymer before prepregging operations, may be likewise encountered. Further, a storage-stable prepreg cannot be made therefrom, which leads to the same attendant difficulties as discussed above.
Therefore, it is the primary object of the present invention to provide a storage-stable advanced polymaleimide composition from which to manufacture storage-stable prepreg products which substantially eliminates the disadvantages encountered with prior art materials.
It is a further object to provide such a storage-stable advanced polymaleimide composition without any substantial adverse impact on the thermal and mechanical properties of the prepreg products made therefrom.
Various other objects and advantages of this invention will become apparent from the following descriptive material.
It has now been surprisingly discovered that the addition of phenothiazine or hydroquinone to the already advanced polymaleimide prepolymer in accordance with the present invention inhibits the further advancement reaction, thereby eliminating or greatly reducing further polymerization and thus, substantially improves the storage-stability of these compositions.
When the phenothiazine or hydroquinone is added is critical to the present invention. If added to the monomer mixture before prepolymerization, it will react with the bismaleimide/diallyl phenol monomers, which results in the phenothiazine being reacted in and hence, not available as an antioxidant in the prepolymer prepreg product. In contrast, the addition of phenothiazine or hydroquinone after advancement surprisingly leads to advanced polymaleimide compositions which can be stored at room temperature for prolonged periods of time without adverse effects to processing conditions, thereby eliminating the problems associated with prior art materials such as the user's need to store such resin systems at low temperatures, i.e., refrigeration conditions.
Furthermore, the addition of the phenothiazine or hydroquinone after advancement surprisingly improves other processing characteristics of the resin system. For example, the pot-life of the resin system in accordance with the present invention is extended improving the processability for the prepreg manufactured therefrom, and the residual volatile content is reduced. Thus, the advanced polymaleimide compositions in accordance with the present invention provide improved coating and processing conditions for higher quality materials.